1. Field of the Invention
The present invention relates to a back light unit, and more particularly, to a prism sheet capable of enhancing light efficiency and viewing angle characteristics, a backlight unit using the prism sheet, and a method for fabricating the prism sheet.
2. Discussion of the Related Art
Recently, various flat panel display devices have been developed that eliminates bulky and heavy structural nature disadvantages of cathode ray tubes (CRTs). One such flat panel display devices is a liquid crystal display (LCD). The LCD displays a desired image by adjusting a transmission amount of light beams irradiated from a backlight unit using a liquid crystal panel including a plurality of liquid crystal cells and a plurality of control switches adapted to switch video signals to be supplied to the liquid crystal cells, respectively.
FIG. 1 is a schematic view illustrating a related art backlight unit.
As illustrated in FIG. 1, the related art backlight unit includes a lamp 10 which emits light, a light guide plate 20 which guides the light incident thereto from the lamp 10, a lamp housing 12 which encloses an incidence face 22 of the light guide plate 20 and the lamp 10, a reflection plate 30 which is arranged beneath the light guide plate 20, a diffusion sheet 40 which is arranged on the light guide plate 20 to diffuse light emerging from the light guide plate 20, and a prism sheet 50 which adjusts the traveling direction of light emerging from the diffusion sheet 40.
For the lamp 10, a cold cathode fluorescent lamp is mainly used. The lamp 10 is turned on by a lamp drive voltage supplied from an inverter (not shown), to irradiate light to be incident to the incidence face 22 of the light guide plate 20 formed at one side of the light guide plate 20.
The lamp housing 12 is arranged at the incidence face side of the light guide plate 20 to enclose the lamp 10 and the incidence face 22 of the light guide plate 20. The lamp housing 12 is provided, at an inner surface thereof, with a reflection face to reflect the light from the lamp 10 toward the incidence face 22 of the light guide plate 20.
The light guide plate 20 enables the incident light from the lamp 10 to reach a position spaced apart from the lamp 10 by a great distance, and guides the incident light toward the diffusion sheet 40. A printed pattern is provided at an inclined lower surface of the light guide plate 20 to form an inclined reflection face, so that the light incident to the incidence face 22 is reflected from the inclined reflection face of the light guide plate 20 at a certain angle such that the light travels uniformly toward the diffusion sheet 40.
The reflection plate 30 is arranged beneath the light guide plate 20 to re-reflect any light incident to thereto from the reflection face of the light guide plate 20 toward the light guide plate 20 to reduce a loss of light.
The diffusion sheet 40 diffuses the light emerging from the light guide plate 20 to the entire region of the prism sheet 50.
The prism sheet 50 condenses the light emerging from the diffusion sheet 40. To this end, the prism sheet 50 includes a condensing film 52 made of polyester (PET), and prism peaks/crests 54 formed on an upper surface of the condensing film 52 in the form of stripes, as shown in FIG. 2.
Each prism crest 54 has first and second inclined faces each having a certain inclination from the apex of the prism crest 54. For example, each of the first and second inclined face is inclined by about 45° from the upper surface of the condensing film 52.
When it is assumed that the condensing film 52 of the prism sheet 50 has a refractive index n1, and the outside of the prism sheet 50 has a refractive index n2, light incident to the condensing lens 52 at an angle θ1 is refracted at an angle θ2 at the prism peaks 54 in accordance with Snell's Law, as expressed by the following equation 1, and is then emitted to the outside of the prism sheet 50.
                                          n            ⁢                                                  ⁢            1                                n            ⁢                                                  ⁢            2                          =                              sin            ⁢                                                  ⁢            θ            ⁢                                                  ⁢            1                                sin            ⁢                                                  ⁢            θ            ⁢                                                  ⁢            2                                              equation        ⁢                                  ⁢        1            
In the above-mentioned related art backlight unit, light emitted from the lamp 10 is advanced toward the diffusion sheet 40 arranged above the light guide plate 20 via the light guide plate 20. The light emitted from the light guide plate 20 is diffused by the diffusion sheet 40 over the entire region of the prism sheet 50. The diffused light is then condensed as it passes through the prism sheet 50, and is then externally emitted.
In the above-mentioned related art backlight unit, the light incident to the prism sheet 50 may be divided into those of three regions, namely, a full reflection region, a condensing region, and a side lobe region, as shown in FIG. 3.
In detail, the light of the full reflection region, namely, light (A) vertically incident to the condensing film 52, is reflected by the first and second inclined faces of the prism peaks 54, so that the light (A) is directed back to the light guide plate 20. Thus, the light (A) is recycled to be condensed.
The light of the condensing region, namely, light B incident to the condensing film 52 at a particular angle, is refracted by the first or the second inclined face of the prism peaks 54, so that the light (B) is condensed and emitted as useful light.
On the other hand, the light of the side lobe region, namely, light (C) incident to the condensing film 52 at another particular angle, is reflected by the first or the second inclined face of the prism peaks 54, so that the light (C) escapes and thus causes degradation in light efficiency and in viewing angle characteristics.
For this reason, there are bright regions in the prism sheet 50 at opposite sides of a symmetrical line of the prism sheet 50, as shown in FIG. 4. Thus, the related art backlight unit has problems caused by the structure of the prism sheet 50, namely, a viewing angle asymmetry caused by brightness asymmetry and a degradation in light efficiency caused by side lobes.
In order to solve the problems incurred in the related art prism sheet, a lenticular lens sheet to be used as a condensing sheet has been proposed. Such a lenticular lens sheet is disclosed in U.S. Pat. No. 5,870,224 issued to Saitoh et al. on Feb. 9, 1999 and is shown in FIG. 5.
Referring to FIG. 5, the related art lenticular lens sheet includes a plurality of lenticular lenses 21 formed on an upper surface of a transparent substrate 1, a reflection pattern 23 formed on a lower surface of the transparent substrate 1, a condensing layer 14 formed on the lower surface to cover the reflection pattern 23, and a transparent resin layer 16 formed on the condensing layer 14. The related art lenticular lens sheet attempts to prevent the loss of light caused by the side lobes in the structure of FIG. 4 using the reflection pattern 23 and lenticular lenses 21.
The reflection pattern 23 of the related art lenticular lens sheet may be formed using the following method. First, an adhesive layer (not shown) is coated over the lower surface of the transparent substrate 1. Parallel light is then irradiated to the transparent substrate 1 from above the lenticular lenses 21 and is condensed at the adhesive layer by the lenticular lenses 21. The adhesive layer loses adhesion properties in regions where the light is irradiated and retains adhesion properties in regions where no light is irradiated. A reflection pattern is transferred to the adhesion layer to form the reflection pattern on portions of the transparent substrate 1 corresponding to respective valleys of the lenticular lenses 21.
The reflection pattern 23 functions to shield and reflect light that would cause a degraded condensation. The light reflected by the reflection pattern 23 is re-reflected by a reflection plate arranged beneath the reflection pattern 23, so that the reflected light is recycled.
However, the above-mentioned related art lenticular lens sheet has a problem in that the reflection pattern 23 bonded to the adhesive layer may deform due to heat or external pressure or may peel off due to a degradation in the adhesion force of the adhesive layer. Furthermore, the related art lenticular lens sheet has a problem in that it is impossible to form the reflection pattern 23 with diverse shapes because the reflection pattern 23 is bonded to the lower surface of the transparent substrate 1 using the parallel light.